The instant invention is directed to papermaking fabrics for use in papermaking machines consisting of a paper forming section where a wet sheet is formed from a dilute suspension of cellulose fibers in water. The dilute fiber suspension of 0.2 to 1.0% solids is discharged at speeds up to 6000 ft./min. on to a moving forming fabric moving at the fiber discharge speed .+-.30 ft./min. As water is removed by suction throughout the permeable forming fabric, a paper sheet is formed on the surface of the forming fabric by a process of filtration and reorientation of suspended fibers to a plane parallel with the plane of the forming fabric surface. The consistency is further increased by additional water removal by suction until it reaches a consistency of 20-25% solids.
At that point, the wet paper sheet has sufficient wet strength to be transferred to the press section where the wet paper sheet is further dewatered by pressing the wet sheet between a smooth roll and a woven or needled papermakers wet felt. In the process of pressing, water is transferred from the wet paper sheet to the papermaker's felt, which is so constructed that it retains the necessary void volume under pressure to pass the water expressed from the wet paper sheet.
Through a series of such presses, the wet paper sheet is subject to increasing pressure and the solid content of the paper will increase from 20-25% to 37-45% depending on paper, grade, machine speed and press construction.
With present technology, 47% solids appear to be the maximum solids content in a sheet of paper that can be obtained through pressing.
After the press section, the paper sheet is transferred to the drying section where the rest of the moisture in the pressed sheet of paper is removed by evaporation in the following manner. First, the paper sheet is brought in contact with the hot surface of a series of rotating steam heated cylinders. These dryer fabrics are made from polymeric material and can be either woven fabric, spiral fabric or other fabric construction. These fabrics are made endless by a cross machine direction seam during installation. The edges of the fabric are subject to damage and wear due to contact with stationery machine parts and guide pedals. It is therefore imperative that the fabric edges be protected. The subject of this invention is an edge coating for a dryer fabric of polymeric material which is degradation resistance and the method for applying this coating.
Dryer fabrics are subject to thermal and chemical degradation by hydrolysis on the paper machines. This degradation is produced by the high temperatures of between 320.degree. F. to 350.degree. F. and high humidity of 0.25 to 0.35 lb. of water per pound of dry air to which it is subjected in use. It is essential that the treatment of the edges of the dryer fabric have a resistance to thermal degradation and degradation by hydrolysis that exceeds that of the polymeric material forming the dryer fabric.
It is usual that dryer fabrics have a sealer coating applied to their edges in order to protect them against wear and prevent unraveling. Sealed edges also act to stabilize the entire dryer fabric against yarn shifts which render the drainage channels uneven when the fabric is in use.
The most common coatings employed as an edge seal are polyurethane and epoxies polyurethanes. These materials have the capability of securely adhering to the monofilaments forming the dryer fabric and they possess good degradation resistance to most of the chemicals employed with papermaking machines in the papermaking process. It has been found, however, that these coatings are not sufficiently degradation resistant for use with dryer fabrics formed of polyphenylene sulfide, polypropylene terephtalate and others which have a useful life of between 300 and 400 days on the paper machine.
A second problem with the current coating materials as an edge seal coating is the cost of application. These materials, when applied as a liquid to the fabric edge, require between twenty minutes and twenty four hours to dry completely. The coating process normally takes place on a circular fabric, which is carried by a pair of drive rolls under tension. Due to the drying time required, the coated edges are passed over the rolls in a partially dried state. In this state, the coating material attaches to the drive rolls. The coating material on the rolls tend to stick with the coating on the fabric edges to produce an uneven bead along the fabric edge. Eventually these attached coatings lead to degradation of the edge seal.
To eliminate this attaching, the drive rolls must be covered with tape at the edge portions of the fabric. The tape is of a material to which the coating material does not readily adhere. This process improves the procedure, but eventually the coating material builds up and the above problems are encountered.
Numerous attempts have been made to find a more satisfactory edge seal coating as indicated by the below referred to U.S. Patents.
U.S. Pat. No. 2,718,791 to Hose et al show one such attempt. Here, the edge portions of a Fourdrinier belt are coated with a polyethylene material which is flexible and elastic. Polyethylene degrades at around 300.degree. F.
U.S. Pat. No. 3,523,867 to MacBean discloses a sealed coated edge for a paper forming fabric similar to the Hose et al patent. MacBean employs nylon, polyethylene or polyvinylchloride as the coating material. The fabric of this patent encounters similar problems to those discussed above.
U.S. Pat. No.3,652,390 to Peterson is also directed to an edge seal coating for a paper forming fabric. Here the edges are coated with polyethylene or rubber.
None of the patents discussed provide a solution for the deficiencies as set forth above.
Accordingly, it is an object of the invention to provide a stable coil constructed dryer fabric for paper forming machines.
Another object of the invention is to provide a dryer fabric capable of operating at temperatures of between 320.degree. and 350.degree. F.
Another object of the invention is to provide an edge seal coating which protects the fabric forming yarns forms the fabric edge and prevents wear and maintains stability.
Another object of the invention is to provide a novel method of coating the edges of dryer fabrics formed of coiled filaments.
Another object of the invention is to provide a method of edge coating and drying a forming fabric between fabric carrying rolls.
Another object of the invention is to provide a more cost efficient method of edge coating a dryer fabric.
Another object of the invention is to provide a polymer paste which may be polymerized into a non-tacky condition immediately after application to the edge portions of a dryer fabric.